Liquid level gauges have many applications. One such important, and exacting, application is the gauging of liquid level in a boiler drum. Exacting because the reliability requirements are understandably high.
Higher degrees of reliability are often achieved by means of systemic redundancy. For instance, in a paper entitled "Hydrastep--An Electronic Boiler Drum Water Level Gauge" by A. A. G. McInnes given at the Institution of Certificated Mechanical and Electrical Engineers, S.A. on Nov. 20, 1975, duplicate water columns and associated circuitry are utilized. Otherwise, the paper describes a gauging system using simple resistance measurement between a probe and the body of water in the container or column.
The system disclosed in U.S. Pat. No. 3,935,739, not being particularly concerned with high reliability, is for a liquid level gauge for monitoring the height of liquid in a storage tank. In the tank is submerged an upright conducting probe. Close to the probe is a series of electrodes along its length. The probe is excited by an a.c. voltage source, and each electrode is connected to one input of a digital comparator, the other input of which is connected to a reference voltage. The idea is that when fluid is intervening between the probe and an electrode, the mutual capacitance increases due to increased dielectric constant and thus also signal coupling.
U.S. Pat. No. 3,916,213 discloses a liquid level control system for sump pit and tank level control utilizing solid state sensing of conductive liquids. A pair of electrodes are disposed in the liquid at the two levels at which level control is desired. The electrical circuitry generates a high frequency signal from the power line frequency and the attenuation of the high frequency signal by the conductive liquid acts to energize circuits.
U.S. Pat. No. 4,099,167 discloses a system wherein electrodes are disposed on portions of a container comprising dielectric material. An electrical signal is capacitively transmitted between the electrodes, and the electrical signal is shunted by the presence of a liquid in close proximity to the electrodes. The shunting is detected.
U.S. Pat. No. 4,170,765 teaches a method and apparatus for indicating the level of liquid in a tank or the like whereby a transducer is mounted on a pipe above the liquid for producing, in response to a trigger signal, an ultrasonic pulse which is transmitted toward and reflected to the transducer first from a calibration target in the pipe above the liquid and then from the liquid so that the transducer produces echo pulses in response to receipt thereof. In one embodiment the calibration echo signal is applied to a logic circuit together with the output of a counter which is incremented to a predetermined count following the trigger signal by pulses from a voltage controlled oscillator so that the logic produces a control signal indicating the difference in time between when the calibration echo is received and when the counter reaches its predetermined count and which event occurs first. The logic then varies the frequency of the oscillator to reduce the difference. A further counter is enabled upon receipt of the calibration echo signal to accumulate a count of pulses from the oscillator and disabled upon receipt of the liquid echo signal so that the count indicates the liquid level.
U.S. Pat. No. 4,125,021 discloses apparatus for detection of the level of a liquid metal. The apparatus has a liquid level detecting probe provided with an exciting coil wound in a closed bottom non-magnetic sheath over the approximately whole length thereof, a level detecting coil wound in the sheath in an upper part thereof, and a temperature compensating coil wound in the sheath in a lower part thereof; a temperature control for maintaining the coils at a constant temperature; a drive circuit for supplying the exciting coil with an alternating current of a determined frequency and amplitude; and a signal processing circuit for subjecting output voltages induced in the liquid level detecting coil and in the temperature compensating coil to linear detection and then determining the ratio of the thus detected outputs, thereby detecting the level of the liquid metal. At the time of operation, the liquid level detecting probe is immersed in the liquid metal, the level of which is to be detected in such a manner that the temperature compensating coil is always located below the level of the liquid metal.
U.S. Pat. No. 3,918,035 teaches an indicating apparatus comprising a plurality of indicator lamps connected to respective outputs of a plurality of voltage comparators. The comparators have a first input connected to a common input supplied with an input voltage related to a quantity to be measured and a second input connected to respective reference voltages whereby such comparator energizes its respective lamp when the input voltage equals its respective reference voltage.
The above methods and apparatuses would not generally be suitable for applications requiring high reliability. This is in part due to excessive sensor complexity or that of the physical effect used for sensing, which may result in less robust systems, if not also expensive ones.
On the other hand, simple sensing and detection methods, such as mere reliance on galvanic resistance measurement, are prone to an uncertainty mitigated only by redundancy, again leading to increased cost.